1. Field of the Invention
The present invention relates to an aluminum nitride wiring substrate and a method for the production thereof and, more particularly, to an aluminum nitride wiring substrate in which a wiring layer can be densified while keeping high thermal conductivity inherent in aluminum nitride, the electric resistance of the wiring layer can be reduced while keeping insulating characteristics of the substrate, and odd appearance or abnormal plating rarely occurs and a method for the production thereof.
2. Description of the Related Art
Recently, the demands for ceramic substrates have been increasing year after year in consequence of the development of such semiconductor elements as power IC's and high frequency transistors which necessitate large volumes of electric current. Particularly, aluminum nitride (AlN) substrates are characterized by high thermal conductivity, an excellent ability to radiate heat, and etc, and, owing to these features, have been attracting attention as substrates capable of efficiently serving semiconductor elements which have been tending toward increasing capacities for heat radiation.
When the aluminum nitride substrate just mentioned is used as in a semiconductor package or a circuit substrate, the practice of collectively manufacturing the aluminum nitride substrate and a wiring metal layer by simultaneous sintering is popularly followed.
The method generally adopted for the production of a simultaneously sintered aluminum nitride substrate will be described below.
First, a wiring layer-forming metallic paste containing a wiring metal is applied in a desired wiring shape to a ceramic green sheet consisting of aluminum nitride. By shaping one layer so obtained above or superposing a plurality of such layers in such a manner as to form a desired shape, an aluminum nitride compact is obtained. In this case, a wiring metal for the aluminum nitride substrate, tungsten is generally used to hold melting resistance properties (heat resistance) and to decrease the difference between the thermal expansion coefficients of the substrate and the wiring layer. Then, this aluminum nitride compact is subjected to a degreasing treatment and thereafter sintered at a prescribed temperature to sinter the aluminum nitride substrate and the wiring metal simultaneously.
In the simultaneously sintered aluminum nitride substrate under discussion, the union or bonding of the wiring metal of tungsten as with the aluminum nitride sintered body is considered to be maintained by the anchoring effect that the composite oxide of a sintering auxiliary agent and aluminum oxide enter each other's textures.
Incidentally, the process for simultaneously sintering the aluminum nitride substrate has the problem that since the sintering temperature of aluminum nitride and the densifying temperature of tungsten are different in behavior of densification, the step of simultaneous sintering tends to leave pores or voids behind in the freshly formed wiring metal layer. It further has the problem that the bonding characteristic between aluminum nitride substrate and wiring metal layer grows unstable. Thus, the practice of incorporating aluminum nitride as a component and a sintering auxiliary component therefor in a printing composition to be used for the formation of a wiring metal layer thereby equalizing the coefficient of contraction of the sintered body of aluminum nitride and that of the wiring metal and, at the same time, causing the aluminum nitride component to eliminate pores has been heretofore followed. As the result, the wiring metal is densified.
However, tungsten constituting the wiring layer disadvantageously serves as a metal having a high electric resistance, so that there is posed a problem that the resistance of the signal wiring layer of an AlN package cannot be reduced. When the aluminum nitride component having high insulating characteristics or the sintering auxiliary component is added to the printing composition of the wiring metal as cited above, if an amount thereof increases, the volume rate of metal tungsten to the wiring layer decreases. For this reason, defective wire continuity or an increase in wiring resistance easily occurs, and an high-speed and high-output element cannot be obtained. Further, when it incorporates therein the aluminum nitride component containing the sintering auxiliary component, it now contains a component destined to give rise to a liquid phase in the wiring metal layer in the process of sintering and the wiring metal layer to be obtained after the sintering tends to induce segregation of a liquid phase on or around the wiring metal layer. The produced substrate, therefore, is at a disadvantage in inducing abnormal plating or odd appearance. When the wiring metal layer has not been fully densified, the problem arises that in the heating test subsequent to the formation of a plating layer or a thin-film wiring layer, etc thereon, the plating liquid filled in voids expands or is gasified, and defective expansion tends to occur in the plating layer or in the thin-film wiring layer, and etc.
On the other hand, with the increase in integration density of a recent semiconductor element, high-density packaging, or miniaturizing of parts, micropatterning of a signal wiring mounted on a semiconductor element proceeds, and the section of the wiring tends to decrease. In order to increase a signal transmission rate to realize high-speed processing, it is proposed as a practical problem that the electric resistance of the wiring layer such as a signal wiring is further reduced.
When a signal response speed increases, and an increase in arithmetic processing amount of the semiconductor element and an increase in scale of the element following it, heat generated by the element increases. A countermeasure for effectively radiating the heat is necessary, but a sufficient means for solving the problem has not been obtained.
Further, the following knowledge has been obtained by the present inventors. Namely, when copper, silver, a copper compound, or a silver compound is added to an AlN green sheet serving as a substrate, and the AlN green sheet and a wiring layer-forming paste applied thereon are simultaneously sintered, a copper alloy or a silver alloy are precipitated in the wiring metal layer. The knowledge that the above is effective to decrease the electric resistance of the wiring metal layer.
However, a copper or silver component is liable to be precipitated in the wring metal layer, and, at the same time, is easily left in an aluminum nitride sintered body. For this reason, the copper or silver compound advantageously decrease the insulating resistance of the entire wiring substrate. On the other hand, when the copper or silver compound is added to a wiring layer-forming paste, densification of the wiring metal layer consisting of tungsten is easily obstructed, and only a wiring substrate having a wiring metal layer having a large number of voids can be obtained.
On the other hand, when manganese or a manganese compound is added to the wiring metal layer, densification of the wiring metal layer is enhanced, but the electric resistance of the wiring metal layer disadvantageously increases. In any cases, there can hardly be obtained a wiring substrate which can completely satisfy the densification and low resistance of the wiring metal layer and the high thermal conductivity and insulating characteristics of the substrate.